1. Field of the Invention
The present invention relates a nitride semiconductor light emitting device comprising a nitride semiconductor represented by general formula of InxAlyGa1-x-yN (0≦x<1, 0≦y<1), and particularly to a nitride semiconductor light emitting device having an n-side pad electrode and a p-side pad electrode formed on the same side of a substrate.
2. Description of the Related Art
A nitride semiconductor light emitting device commonly used is constituted by forming at least an n-type nitride semiconductor layer and a p-type nitride semiconductor layer one on another on a substrate made of sapphire, SiC, GaN or the like. The p-type nitride semiconductor layer has a p-side pad electrode formed thereon for connecting to a positive terminal of an external power source, and the n-type nitride semiconductor layer has an n-side pad electrode formed thereon for connecting to a negative terminal of the external power source. Electric current flows from the p-side pad electrode to the n-side pad electrode, so as to cause light emission.
In case an insulating substrate such as sapphire is used, the n-side pad electrode cannot be formed on the back surface of the substrate. Therefore the p-type nitride semiconductor layer and a part of the n-type nitride semiconductor layer are removed so as to expose the n-type nitride semiconductor layer on the top side, and the n-side pad electrode is formed thereon.
Also, because the p-type nitride semiconductor layer has a sheet resistance higher than that of the n-type nitride semiconductor layer, a translucent electrode is often formed between the p-side pad electrode and the p-type nitride semiconductor layer in order to assist the diffusion of current in the p-type nitride semiconductor layer (refer to, for example, Japanese Patent Unexamined Publication (Kokai) No. 6-338632). The translucent electrode is formed over substantially the entire surface of the p-type nitride semiconductor layer, from a translucent material such as thin metal film so as to spread the current throughout the p-type nitride semiconductor layer and not to block light emission.
Japanese Patent Unexamined Publication (Kokai) No. 2000-164930 proposes to improve the current distribution by forming linear extensions from the n-side pad electrode and from the p-side pad electrode, on a nitride semiconductor device having the n-side pad electrode and the p-side pad electrode formed on the same side of a substrate. FIG. 9 is a plan view showing an example of the nitride semiconductor device disclosed in Japanese Patent Unexamined Publication (Kokai) No. 2000-164930. In this example, an extension 12a is formed to extend from the n-side pad electrode 12 and surround the device in a C shape, so as to improve the distribution of current which flows from the p-side pad electrode 16 to the n-side pad electrode 12.
In the nitride semiconductor light emitting device having the constitution described above, uniformity of current distribution within the device plane deteriorates when the p-side and n-side electrodes are not well-balanced in the resistance to the current flowing along the surface (substantially determined by the balance of sheet resistance between the translucent electrode formed on the p-type nitride semiconductor layer and the n-type nitride semiconductor layer). Particularly in case the translucent electrode is formed from an electrically conductive oxide such as indium tin oxide (hereinafter referred to as “ITO”), the translucent electrode tends to have higher sheet resistance than that of the n-type nitride semiconductor layer. As a result, such a problem can easily occur that current flows more around the p-side pad electrode and less around the n-side pad electrode. Uneven current distribution within the element plane may lead to a problem related to the electrical characteristic such as higher value of Vf (forward voltage).
Also, the problem described above becomes more significant as the shape of the device becomes more proximate to rectangle. This is because a device having rectangular shape has longer path from the p-side pad electrode to the n-side pad electrode than a square device having the same surface area.
This problem may be addressed by forming linear extensions which extend from the n-side pad electrode and the p-side pad electrode as described in Japanese Patent Unexamined Publication (Kokai) No. 2000-164930. However, the n-side pad electrode and the p-side pad electrode, including the extensions thereof, have the property of blocking light, and absorb light. Thus simply forming the linear extensions from the n-side pad electrode and from the p-side pad electrode leads to smaller area of light emission as the extensions become longer, thus resulting in a decrease in the efficiency of light emission due to the absorption of light.